This invention relates generally to highlight color imaging and more particularly to a printing apparatus and method for forming one black and two color images.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, not only are the charged (i.e., unexposed) areas developed with toner but the discharged (i.e., fully exposed) images are also developed. Thus, the charge retentive surface contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-exposed (i.e. charged) areas of the photoreceptor, as in the case of conventional xerography, while the other image areas correspond to fully exposed (i.e., discharged) areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by Charged-Area-Development, i.e. CAD) remains at or near the fully charged photoreceptor potential represented by V.sub.cad or V.sub.ddp as shown in FIG. 1a. The other images are formed by discharging the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by Discharged-Area-Development (DAD). The background areas are formed by discharging the photoreceptor to reduce its potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased (V.sub.bb, shown in FIG. 1b) about 100 v closer to V.sub.cad than V.sub.white is to V.sub.cad, resulting in a V.sub.bb of about 600 volts, and the DAD developer system is biased (V.sub.cb, shown in FIG. 1b) about 100 v closer to V.sub.dad than V.sub.white is to V.sub.dad resulting in a V.sub.cb of about 400 volts.
As developed, the composite tri-level image initially consists of both positive and negative toners. To enable conventional corona transfer, it is necessary to first convert the entire image to the same polarity. This must be done without overcharging the toner that already has the correct polarity for transfer. If the amount of charge on the toner becomes excessive, normal transfer will be impaired and the coulomb forces may cause toner disturbances in the developed image. On the other hand, if the toner whose polarity is being reversed is not charged sufficiently its transfer efficiency will be poor and the transferred image will be unsatisfactory.
The non-image, or white, or background potential of a conventional tri-level image is of extreme importance in the multi-level imaging contemplated by the present invention. For example, it can be used to form a second DAD image. The exposure step for applying the second color image in a DAD mode, in accordance with the present invention, is done with an LED, a vacuum fluorescent (VF), or a liquid crystal (LX) array. These arrays are typically more compact than laser scanners, but suffer from the drawback of being less uniform in their exposure characteristics than the laser scanner. Thus, these exposure systems lead to wide variations in the background potential, which subsequently require large cleaning fields to suppress background development. As the total potential available for development of the image is set by characteristics of the photoreceptor, the requirements for large cleaning fields reduce the potential available for the latent image.
An additional problem with using more than one exposure step is the registration of one image with respect to another on the same printed page. Systems which use one exposure step for each color will have images displaced from the ideal position due to variations in photoreceptor velocity between one image step and the next. An example of such a system is disclosed in U.S. Pat. No. 4,403,848 granted to Snelling on Sept. 13, 1983. As disclosed therein the production of multiple-color images is effected by means of exposing and subsequently developing a multiplicity of DAD images prior to transfer to paper. Each image requires an exposure step.
Another example of a system requiring one exposure step for each image is disclosed in U.S. Pat. No. 4,562,130 granted to Oka on Dec. 31, 1985. Oka discloses the production of a two-color image derived from a positive optical image and an electronic image. Due to the inexactness of the background potential after the first exposure, a precision recharging mechanism is required in order to level the potential in the non-image area of the photoreceptor prior to exposure by the second electronic imaging source. Again, as in the case of Snelling's device, one exposure step is required for each color on the printed page. This latter point is significant, in that, the more exposure steps used the more difficult it is to effect acceptable image registration.
In addition to the image registration problem, imaging systems which employ multiple exposure steps require that the electronic form of the image be delayed a period of time determined by the distance between exposure stations and the velocity of the photoreceptor. In electronic printing systems which have different information on every single page, the precise coordination of these delays and the buffering of electronic information between exposure steps in an extremely difficult task.
A multiple color imaging system which do not require an exposure step for each image is known. For example, highlight color imaging as taught by Stark in U.S. Pat. No. 4,731,634 issued Mar. 15, 1988 uses a single exposure to create a four level composite latent image. Because there is only one exposure, the composite parts of the latent image are in perfect registration. The image therein is formed using a quad level raster output scanner. The disadvantage of the quad imaging approach is that the development contrast available for each color is less than V.sub.0 /4. Moreover, two of the four images are formed by one of the CAD and one of the DAD images being over-printed by its companion CAD or DAD color.